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Fixed Link 5.8 GHz Could Be Impacted By FCC 5 GHz Band Expansion

The FCC has posted the Notice of Proposed Rulemaking (FCC 13-22) regarding new rules and new spectrum for unlicensed 5 GHz devices. A close reading of the Notice of Proposed Rulemaking indicates the changes could impact unlicensed fixed links some broadcasters are using under Part 15.247 in the 5.725-5.850 GHz band.

These devices, operating under Section 15.247, have the same 1 Watt conducted power restriction as U-NII devices, but higher radiated power levels for point-to-point operations since this rule section don’t mandate a power penalty for high antenna gains. One way stations achieve higher effective radiated power is by coupling the 5.8 GHz signal into existing high gain 7 GHz microwave antennas.

The rule disparities allow manufacturers of 5 GHz unlicensed devices to license some frequency bands under the U-NII rules in Section 15.407, but under less restrictive Section 15.247 rules in the 5.725-5.850 GHz part of the band. This has caused problems, as the FCC notes in the NPRM:

“Most frequently we have seen devices certified to operate in the 5.725-5.85 GHz band with higher power levels and later modified to enable operation in the U-NII-2 frequency bands. These device modifications have resulted in non-compliant devices creating interference scenarios that were not anticipated when the U-NII rules were created.”

The FCC proposed two changes to deal with the disparity. It would first extend the U-NII-3 band from 5.825 GHz to 5.85 GHz to match the amount of spectrum available for digitally-modulated devices under Section 15.247. It would also propose consolidation of all equipment authorizations for digitally-modulated devices in the 5.725-5.85 GHz band under the U-NII rules, but maintain many of the technical rules that made equipment authorization under 15.247 more attractive for equipment manufacturers.

Reading the NPRM, it appears existing devices will continue to be authorized, but transmitter power may have to be reduced to comply with the proposed rule. In most cases in this band the FCC kept the emission bandwidth and power spectrum density limits the same as under 15.247 and would apply these restrictions to U-NII devices operating in the band under Section 15.407. However, under the proposed rules, users of point-to-point systems operating under Part 15.247 would have to reduce power by 1 dB for each dB of antenna gain above 23 dBi. Considering that most 7 GHz dishes have gains around 40 dBi or more, the required power reduction for 5.8 GHz systems coupled to such antennas may be significant, depending on existing system losses in the coupler and waveguide. The new rules would apply the more restrictive unwanted emission limits in Section 15.407 to digitally modulated devices currently operating 15.247 rules. Systems would also be subject to the 13 dB peak-to-average power ratio under Section 15.407.

I've only covered the portion of the NPRM likely to impact broadcasters. The changes to the 5 GHz U-NII (Unlicensed National Information Infrastructure) bands should have a very positive impact on the use of this band for “Wi-Fi” devices, including new devices operating under IEEE Standard 802.11ac with bandwidths up to 160 MHz and data rates of approximately 1 Gbps. The U-NII spectrum includes bands shared with weather radar and other services. The NPRM does a good job explaining the existing and proposed bands and rules and covers some of the history of the band as well. For a NPRM, it is actually interesting reading.